Sorry, I don't know what you mean by "change page"
There is two views in FormsCentral. A Page View (for PDF) and a Web View (for HTML forms). If you want to insert a page break in the PDF you need to make sure you are on the Page View. To change the View look in the bottom right corner of the application you will see a dropdown that looks like this :
Make sure you select "Page View" then insert your "Page Break"
Hope this helps